Methods for marking a sintered product and for fabricating magnetic head substrate

ABSTRACT

A marking method for a sintered product of the invention includes forming a concave portion on the sintered product by irradiating the sintered product with laser light thereby to write identification information on the sintered product. The depth of the concave portion is adjusted in a range between 0.1 μm and 5 μm, inclusive.

BACKGROUND OF THE INVENTION

The present invention relates to a method for marking a sintered productwith identification information while minimizing the generation ofcontamination. The present invention also relates to a method forfabricating a substrate for a magnetic head including the marking step,a sintered product with identification information marked thereon, amagnetic head, and a storage medium drive.

In recent years, thin film magnetic heads of various constructions havebeen used as magnetic head sliders for hard disk drives (HDD), tapestorages, floppy disk drives (FDD), and the like. Sintered substrates,made of compositions such as Al₂O₃—TiC, SiC, and ZrO₂, are used for suchthin film magnetic heads.

FIG. 1A illustrates a typical thin film magnetic head slider 10. Themagnetic head slider 10 has two side rails 11 on the track side thereoffacing the surface of a magnetic disk. The surface of the magnetic headslider 10 where the side rails 11 are formed is sometimes called an airbearing surface (ABS). When the magnetic disk is rotated at high speedwith a motor or the like, in the state where the side rails 11 of themagnetic head slider 10 lightly press the surface of the magnetic diskunder head suspension, a layer of air formed on the surface of themagnetic disk intrudes into a gap under the air bearing surface of theslider 10. This causes the magnetic head slider 10 to be slightlylifted. In this way, the magnetic head slider 10 hovers near the surfaceof the magnetic disk, to effect recording/reproducing operation.

Thin films 12 are formed on one end face of the magnetic head slider 10for magnetic interaction with a storage medium such as the magneticdisk. The thin films 12 constitute an electric/magnetic transducerelement. On the other end face of the magnetic head slider 10,identification information 13, such as a serial number, is engraved.Methods for engraving the identification information 13 on the sinteredsubstrate are disclosed in Japanese Laid-Open Patent Publication No.9-81922, No. 10-134317, and No. 11-126311, for example.

The magnetic head slider 10 is obtained in the following manner. A bar20 as shown in FIG. lB is cut from a sintered substrate 1 as shown inFIG. 1C, and the bar 20 is divided into a plurality of chips. Thepositional relationship between the sintered substrate 1 shown in FIG.1C and the magnetic head slider 10 shown in FIG. 1A is such that an endface 4 of the sintered substrate 1 is parallel with the air bearingsurface of the magnetic head slider 10.

As the size of thin film magnetic heads 12 is reduced to cope with thereduction in size and weight of electronic apparatuses, the thickness ofthe sintered substrate 1 (corresponding to the length L of the magnetichead slider 10) is reduced, and the thickness T of the bar 20(corresponding to the height of the magnetic head slider 10) is reduced.For example, in a magnetic head slider called a pico-slider, L is about1.2 mm and T is about 0.3 mm. With such a short magnetic head slider,the size of characters to be engraved thereon should also be madesmaller.

A laser marking method is commonly used for engraving (hereinafter, alsocalled “inscription” and “marking”) of the identification information13. According to the laser marking method, the identificationinformation 13 shown in FIGS. 1A and 1B is inscribed on the back surface3 of the substrate 1 in the wafer state before being divided into thebars 20. Subsequently, various thin films 12 are formed on the oppositesurface 2 of the substrate 1.

The conventional laser marking method will be described with referenceto FIG. 2. The back surface 3 of the sintered substrate 1 is irradiatedwith a laser beam 6 converged by a lens 5. An irradiated portion of thesubstrate 1 is rapidly heated and evaporated, so that a small concaveportion or recessed portion is formed on the back surface 3 of thesubstrate 1. At this time, pieces of the sintered material constitutingthe substrate 1 scatter. Some of these pieces fall back on the substrate1. By scanning the back surface 3 of the substrate 1 with the laser beam6, an arbitrary recess pattern can be formed. Thus, by forming a patternof alphabets, numerals, barcodes, and the like, various types ofidentification information can be written at arbitrary positions on thesubstrate 1.

There are problems associated with the conventional laser markingmethod. First, debris is generated by the laser light irradiation. Thisdebris often causes contamination in later fabrication process steps, bybecoming absorbed and accumulated in inscribed grooves and the like.Second, burrs are generated at edges of the grooves during the laserlight irradiation. Therefore, an additional step for removing such burrsis required.

FIG. 3A schematically illustrates the cross-section of the sinteredsubstrate 1 after the substrate 1 has been engraved by the conventionallaser marking method. This cross-sectional view was drawn based on aphotograph taken with a scanning electron microscope (SEM). Referring toFIG. 3A, a deep concave portion 30 is formed on the surface of thesubstrate 1 by irradiation with laser light. The depth of the concaveportion 30, measured from the back surface of the substrate 1 in thedirection shown by the arrow a, is 30 to 50 μm. A convex portion (burr)31 is formed along the edge of the concave portion 30. The height of theburr 31 measured in the direction shown by the arrow b is of the orderof several micrometers. The width of the concave portion 30 is of theorder of 20 μm, for example. Hereinafter, the depth of the concaveportion is referred to as the “inscription depth”, and the height of theraised portion around the concave portion is referred to as the “edgeburr height”.

A number of particles 32 attach to the wall of the deep concave portion30 formed by the laser light irradiation. The particles 32 are notnecessarily in the form of particles, but are herein called “particles”for simplification. In order to remove the particles 32 from thesubstrate 1, a long-duration cleaning step, such as ultrasonic cleaning,is required after the marking step. Even by this cleaning step, however,it is difficult to remove the particles 32 located deep inside theconcave portion 30 to a sufficient degree.

If a large number of particles 32 are generated during the laser markingstep, part of the particles 32 may be dispersed in a cleaning Solution,and part of the particles 32 in the cleaning solution may possiblyre-attach to the surface of the substrate 1 that has not been irradiatedwith laser light (surface 2). If this re-attachment occurs and aninsulating thin film made of amorphous aluminum oxide or the like isdeposited on the surface 2 of the substrate 1, the particles 32 may beincluded in the insulating thin film. The surface of the insulating thinfilm is smoothed before a magnetic thin film is deposited thereon.Therefore, if the particles 32 exist in the insulating thin film, theinsulating thin film may be locally peeled off together with theparticles 32, resulting in formation of pinholes in the insulating thinfilm. Even if formation of pinholes is evaded, the insulating thin filmmay be substantially thinned in some portions due to the existence ofthe particles 32. The insulating property of such portions of theinsulating thin film is decreased. Such inclusion of the particles inthe insulating thin film does not necessarily occur. However, as long asthe inscribed portions of the back surface of the substrate serve as adust source, the yield may be lowered in the subsequent steps, and alsothe reliability of the final products may be decreased.

In order to improve the production yield of thin film magnetic heads,the quality of the insulating film deposited on the sintered substrate 1should preferably be improved as much as possible. In order to achievethis, the marking step that is a cause of generation of dust orcontamination should desirably be improved to minimize generation ofdust. In addition, the resultant magnetic head as a complete componentshould not generate dust. Since the magnetic head is used in a cleanenvironment, generation of dust will cause a problem.

A method for replacing the above laser marking method has also beenproposed, where identification information is written on a sinteredsubstrate by chemical etching. However, by this chemical etching method,also, particles enter a concave portion (groove) formed on the substratesurface if the concave portion is deep, resulting in generation of dustor contamination.

An object of the present invention is to provide a marking method for asintered product that can reduce the generation of dust and minimize theformation of burrs.

Another object of the present invention is to provide methods forfabricating with high yield a sintered product, a magnetic headsubstrate, a magnetic head, and a storage medium drive with high qualityby executing an inscribing step according to the above marking method.

SUMMARY OF THE INVENTION

The marking method for a sintered product of the present inventionincludes forming a concave portion on the sintered product byirradiating the sintered product with laser light to writeidentification information on the sintered product, wherein the depth ofthe concave portion is in a range between 1 μm and 5 μm, inclusive.

The method for fabricating a magnetic head substrate of the presentinvention includes the steps of: (1) writing identification informationon a first surface of the magnetic head substrate by the above markingmethod for a sintered product; and (2) subjecting the magnetic headsubstrate to ultrasonic cleaning.

In a preferred embodiment, the method further includes the step offorming a thin film on a second surface of the magnetic head substrateafter the step of subjecting the substrate to ultrasonic cleaning.

The magnetic head substrate of the present invention is marked withidentification information by laser light irradiation, wherein theidentification information is constructed of a concave portion having adepth in a range between 0.1 μm and 5 μm, inclusive.

The magnetic head of the present invention is marked with identificationinformation by laser light irradiation, wherein the identificationinformation is constructed of a concave portion having a depth in arange between 0.1 μm and 5 μm, inclusive.

Alternatively, the marking method for a sintered product of the presentinvention includes the steps of: (1) preparing a sintered product formedof a powder mixture including first powder particles of a first materialand second powder particles of a second material having an etchingcharacteristic different from the etching characteristic of the firstmaterial; and (2) performing selective etching for a selected portion ofa surface of the sintered product, the selective etching includingetching the first powder particles with priority over the second powerparticles, thereby to write identification information on the surface ofthe sintered product.

In a preferred embodiment, the mean grain diameters of the first powderparticles and the second powder particles are in a range between 0.3 μmand 5.0 μm, inclusive.

Preferably, the difference in reflectance between the portion of thesurface of the sintered product that has been subjected to the selectiveetching and a portion that has not been subjected to the selectiveetching is 15% or more for light having a certain wavelength.

The wavelength is preferably included in a wavelength range for lightused for irradiating the sintered product for optically reading theidentification information.

Preferably, the plane roughness of the portion of the surface of thesintered product that has not been subjected to the selective etching is5 nm or less.

Preferably, the mean etching depth of the portion of the surface of thesintered product that has been subjected to the selective etching is ina range between 5 nm and 200 nm, inclusive.

The first material and the second material are preferably compoundsselected from the group consisting of aluminum oxide, aluminum nitride,silicon oxide, silicon nitride, zirconium oxide, zirconium nitride,silicon carbide, titanium carbide, titanium oxide, and iron oxide.

Alternatively, the method for fabricating a magnetic head substrate ofthe present invention includes writing identification information on thesintered product by the above marking method for a sintered product.

The method for fabricating a magnetic head of the present inventionincludes fabricating a magnetic head, provided with the identificationinformation from the magnetic head substrate fabricated by the abovemethod, for fabricating a magnetic head substrate.

The sintered product of the present invention is formed of a powdermixture including first powder particles of a first material and secondpowder particles of a second material having an etching characteristicdifferent from an etching characteristic of the first material, whereinthe first powder particles are selectively etched with priority over thesecond power particles in a selected portion of a surface of thesintered product, to write identification information on the surface ofthe sintered product.

In a preferred embodiment, the mean grain diameters of the first powderparticles and the second powder particles are in a range between 0.3 μmand 5.0 μm, inclusive.

Preferably, the difference in reflectance between the portion of thesurface of the sintered product that has been etched and a portion thathas not been etched is 15% or more for light having a certainwavelength.

The wavelength is preferably included in a wavelength range for lightused for irradiating the sintered product for optically reading theidentification information.

Preferably, the plane roughness of the portion of the surface of thesintered product that has not been etched is 5 nm or less.

Preferably, the mean etching depth of the portion of the surface of thesintered product that has been etched is in a range of 5 nm and 200 nm,inclusive.

The first material and the second material are preferably compoundsselected from the group consisting of aluminum oxide, aluminum nitride,silicon oxide, silicon nitride, zirconium oxide, zirconium nitride,silicon carbide, titanium carbide, titanium oxide, and iron oxide.

Alternatively, the magnetic head substrate of the present invention isformed of the above sintered product.

Alternatively, the magnetic head of the present invention includes: theabove magnetic head substrate; and an electric/magnetic transducerelement formed on the magnetic head substrate.

The storage medium drive of the present invention includes: the abovemagnetic head; a storage medium having a magnetic recording film on/fromwhich information is recorded/reproduced via the magnetic head; and amotor for driving the storage medium.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings an embodimentthat is presently preferred. It should be understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown. In the drawings:

FIG. 1A is a perspective view of a magnetic head slider,

FIG. 1B is a perspective view of a bar from which the magnetic headslider is to be separated, and

FIG 1C is a perspective view of a rectangular sintered substrate;

FIG. 2 is a schematic view illustrating the laser marking step;

FIG. 3A is a cross-sectional view of an inscribed portion deeplyengraved by the conventional laser marking method, and

FIG. 3B is a cross-sectional view of an inscribed portion shallowlyengraved by a marking method of the present invention;

FIG. 4 is a graph showing the relationship between the inscription depthby laser marking and the number of particles;

FIG. 5A is a cross-sectional view of an inscribed portion formed by aconventional marking method using non-selective chemical etching, and

FIG. 5B is a cross-sectional view of an inscribed portion formed by amarking method of the present invention;

FIGS. 6A, 6B, and 6C are cross-sectional views illustrating the steps ofselective etching for marking, and FIG. 6D is a top view illustrating anetched surface and a non-etched surface of a ceramic substrate;

FIG. 7 is a cross-sectional view of a storage medium drive of thepresent invention;

FIG. 8 is an SEM photograph of a marking pattern in Example 2;

FIG. 9 is an SEM photograph of a marking pattern in Example 4;

FIG. 10 is an SEM photograph of a marking pattern in Comparative Example1; and

FIG. 11 is an SEM photograph of a marking pattern in Comparative Example5.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

Embodiment 1

Marking Method for Sintered Product

First, a sintered substrate produced by a conventional method isprepared. The material and composition of the sintered product used forapplying the present invention can be arbitrarily selected.

The sintered product is irradiated with laser light in the manner shownin FIG. 2, to form a concave portion or recessed portion on the sinteredsubstrate. In this step, the substrate surface is scanned with laserlight by driving a support mechanism that holds an optical system of alaser irradiation device (laser marker) or the substrate in X-Ydirections with high precision. During the scanning, the beam spot ofthe laser light moves on the substrate surface according to a programpre-stored in a memory of a computer or the like, thereby formingconcave portions of a given pattern on the substrate surface. In thisway, visually recognizable identification information is written on thesintered product.

As a laser light source, a gas laser such as a CO₂ laser and a YAG laseris preferably used in consideration of the power. The wavelength of theirradiated laser light may preferably be in such a wavelength range,that laser light having a wavelength in this range can be efficientlyabsorbed into the sintered product and rapidly heat the sinteredproduct.

The optical system of the laser is designed so that the beam spotdiameter of the laser light on the surface of the sintered product isroughly as large as the wavelength of the laser light. Although varyingdepending on the size of a code or character to be inscribed, the widthof the concave portion actually formed is of the order of severalmicrometers, which is larger than the beam spot diameter of the laserlight. This is due to the fact that, when the sintered body isirradiated with the laser light, not only the irradiated portion, butalso the surrounding portions, are heated and scattered, resulting information of a relatively widened concave portion.

According to the present invention, the depth of the thus-formed concaveportion is adjusted in the range between 0.1 μm and 5 μm, inclusive.This adjustment of the depth of the concave portion is realized bycontrolling the power of the laser light. Substantially the same effectmay also be obtained by controlling the speed of scanning of the laserlight. However, controlling the power of the laser light is preferableif high-precision control is desired. The reason why the depth of theconcave portion should preferably be adjusted in the range between 0.1μm and 5.0 μm, inclusive, will be described hereinafter in detail.

Method for Fabricating Magnetic Head Substrate

An embodiment of the method for fabricating a magnetic head substrate ofthe present invention will be described.

First, a sintered substrate made of alumina titanium carbide (Al₂O₃+TiC)or the like, for example, is prepared. Vickers hardness of the substrateis 1600 or more. Identification information is inscribed on the surfaceof the substrate opposite to the surface on which magnetic thin filmsare to be formed, that is, the back surface of the substrate, by thelaser marking method described above. The depth of the inscribed groovesis controlled in the range between 0.1 μm and 5 μm, inclusive.

In order to remove particles (dust) attaching during the inscribingstep, the resultant substrate is immersed in pure water (i.e., deionizedwater) for ultrasonic cleaning. Subsequently, an amorphous aluminumoxide film (thickness: 0.5 μm to 20 μm, for example) is deposited on thefront surface. of the substrate by sputtering. The surface of theamorphous aluminum oxide film is then smoothed. Thus, the basic stage ofthe process for fabricating a substrate for a thin film magnetic head iscompleted.

Thereafter, various magnetic thin films are formed on the resultantsubstrate for a thin film magnetic head. The thus-fabricated substrateis divided into a plurality of bars 20 as shown in FIG. 1C. Each of thebars 20 obtained from the substrate has a plurality of marks ofidentification information 13 as shown in FIG. 1B. Each of the magnetichead sliders obtained from each bar 20 has the identificationinformation 13 unique to the magnetic head slider as shown in FIG. 1A.In this way, the serial number of each magnetic head slider can beeasily identified. This makes it possible to perform the processmanagement during fabrication of magnetic heads, as is performedconventionally.

Thus, in this embodiment, a magnetic head having the followingadvantages can be fabricated. That is, the trouble of contaminationgenerated during the laser marking step is overcome. In addition, theresultant magnetic head can be favorably used for a magnetic storagemedium drive that should be protected from dust contamination asstrictly as possible. As a result, the production yield of the magneticheads improves, and the reliability of a magnetic recording/reproducingapparatus provided with the magnetic head improves.

EXAMPLE

In the example of this embodiment, a composite sintered alumina titaniumcarbide substrate made of 66 wt. % of Al₂O₃ and 3 wt. % of TiC was usedas the object (sintered product) to be inscribed with laser light. Thissintered substrate is a machined, thin rectangular plate (50 mm×50 mm)having a thickness of 1.2 mm with a surface thereof beingmirror-finished to have a surface roughness Ra of 0.5 to 1.5 nm usingdiamond slurry (mean grain diameter: 1 μm).

In this example, identification information was inscribed by irradiatingthe back surface of the substrate with laser light having a wavelengthof 1,063 nm. In this inscription, a 7-digit code (composed of sevencharacters and/or numerals) was marked for each head portion. A total of3,000 codes (21,000 characters and/or numerals) were marked on the backsurface of one substrate. The size of each character/numeral engravedwas about 100 μm×150 μm.

A YAG (Yttrium-Aluminum-Garnet) laser was used as the laser lightsource. The scanning speed (marking speed) of the laser light for eachsample was fixed at 40 nm/sec. The depth of the grooves formed bymarking (inscription depth) was controlled by adjusting the opticalpower of the laser light in the range of 0.1 to 1.0 W. The beam spotdiameter of the laser light on the substrate surface was about 10 μm.

The portion of the back surface of the alumina titanium carbidesubstrate irradiated with the laser light was instantaneously heated toa temperature of the melting point or higher, forming a concave portionhaving a width of several micrometers together with a slightly raisedportion surrounding the concave portion. FIG. 3B illustrates in sectionthe portion of the alumina titanium carbide substrate irradiated withthe laser light in this example.

The alumina titanium carbide substrate marked under the above conditionsof laser light irradiation was then subjected to ultrasonic cleaningwith pure water (i.e., deionized water). Thereafter, the number ofparticles (diameter: 2 μm or more) left unremoved on the substrate wascounted with a liquid particle counter. Also, the height of edge burrswas measured under SEM observation. Further, whether or not theinscribed identification information was visually recognizable wasevaluated.

The results of the above experiments are shown in Table 1 below.

TABLE 1 Number of Inscription depth Visual evaluation Edge burr heightparticles 30 μm Recognizable 2.3 μm 489652 10 μm Recognizable 1.5 μm264371 5 μm Recognizable 0.2 μm  8206 3 μm Recognizable 0.2 μm  8004 1μm Recognizable 0.1 μm  6123 0.5 μm Recognizable <0.01 μm  6234 0.1 μmRecognizable <0.01 μm  5261 0.05 μm Not recognizable <0.01 μm  5014

The “number of particles” in Table 1 refers to the number of particlesdetected in 100 ml of pure water. The inscription depth of 0.05 to 30 μmcorresponds to the laser light power of 0.1 to 1.0 W. The relationshipbetween the “number of particles” and the “inscription depth” is shownin the form of a graph in FIG. 4 for the sake of clarity.

As is apparent from Table 1 and FIG. 4, as the inscription depth islarger, the number of particles and the edge burr height increase. Inparticular, when the inscription depth is 10 μm or more, the number ofparticles exceeds 250,000 and the edge burr height reaches 1.5 μm. Onthe contrary, when the inscription depth is 5 μm or less, the number ofparticles is less than 10,000 and the edge burr height decreases to 0.2μm or less.

From the above experiment results, it is found that the inscriptiondepth is preferably 5.0 μm or less. However, if the inscription depth is0.05 μm, the identification information is not recognizable. Therefore,the lower limit of the inscription depth is preferably 0.1 μm or more.From the standpoint of dust reduction, the inscription depth ispreferably 1 μm or less. From the standpoint of keeping high thereadability, the inscription depth is preferably 0.5 μm or more.

The number of particles when the inscription depth is 10 μm is greatlydifferent from that when it is 5 μm. Concretely, the ratio of the formerto the latter is about 30:1. However, when the inscription depth is inthe range between 0.1 μm and 5 μm, inclusive (the range according to thepresent invention), neither the number of particles nor the edge burrheight change so largely. The reason for this is unknown, but it iscertain that by only reducing the inscription depth to 5 μm or less,both the number of particles and the edge burr height can be greatlyreduced.

In the conventional laser marking technique, it has been taken forgranted that the inscription depth should be 30 μm or more probablybecause the highest priority has been given to easy visual recognition.The inventors of the present invention have found that visualrecognition is still obtained even if the inscription depth is 5 μm orless as long as it is within a certain depth range, and, moreover insuch a depth range the number of particles and the edge burr height canbe greatly reduced beyond expectation. The present invention has beenconceived from these findings.

In this embodiment, the sintered product is inscribed by laser lightirradiation. This makes it possible to write identification informationon the sintered product efficiently in a short time even if the sinteredproduct has an extremely high hardness (Vickers hardness of 2000 ormore).

Embodiment 2

If the depth of the engraved concave portion is made too small in orderto solve the problem of contamination due to the engraving of thesintered product, it may sometimes be difficult to read identificationinformation such as engraved letters. In general, the identificationinformation is read by irradiating the engraved portion with light andsubjecting the light reflected from the engraved portion to imageprocessing to recognize the identification information. The reason whythe identification information can be reproduced from the reflectedlight is that contrast in reflected light is generated between theengraved concave portion and the flat portion (background portion). Thiscontrast is higher as the engraved concave portion is deeper. For thisreason, if the engraved concave portion is made shallow in an attempt toreduce contamination due to the engraving, the contrast in reflectedlight will become so low that the engraved identification informationwill fail to be correctly recognized by a conventional wafer characterrecognition system.

FIG. 5A illustrates, in section, a concave portion engraved by aconventional etching method. In FIG. 5A, also illustrated in section isa sintered substrate 40 formed of two kinds of powder particles havingdifferent etching characteristics. Referring to FIG. 5A, on the surfaceof the sintered substrate 40, an etched surface 42 has a width indicatedby C (width of an inscribed groove). The etched surface 42 is located ata lower level than a non-etched surface 41. A steep step is formedbetween the non-etched surface 41 and the etched surface 42.

In the conventional etching method, etching proceeds uniformly even whenthe sintered substrate 40 is formed of two kinds of powder particleshaving different etching characteristics. As a result, a recessedportion having a substantially flat bottom surface is formed on thesintered substrate 40. This non-selective etching generates nosubstantial difference in reflectance between the flat portion(non-etched surface 41) and the bottom surface of the recessed, portion(etched surface 42). In this state, the contrast in reflected lightdepends on the depth of the recessed portion. In order to ensure acontrast required for recognizing the identification information,therefore, it is necessary to form a concave portion having a depthexceeding 200 nm.

Conventionally, the above non-selective etching has been adopted becauseit is necessary to form a deep concave portion efficiently at a highetching rate. In order to form a deep concave portion at an increasedetching rate, the non-selective etching is inevitably adopted.

FIG. 5B illustrates in section a concave portion or recessed portionformed by a marking method of the present invention. In the exampleillustrated in FIG. 5B, selective etching is performed for a sinteredsubstrate 50 formed of two kinds of powder particles (grains) havingdifferent etching characteristics. As a result of the selective etching,one kind of powder particles having a relatively high etching rate(hatched particles in FIG. 5B) are etched with priority over the otherkind of powder particles having a relatively low etching rate(non-hatched particles). As a result, the etched surface 42 has a finerconcave/convex profile. The respective concave and convex portions areminute having a size roughly as large as the size of the powderparticles. The depth of the recessed portion varies depending on theetching time.

In the selective etching, the etching rate A for the material(particles) etched with priority is preferably higher by 10% or morethan the etching rate B for the material (particles) that is relativelynot easily etched. In other words, preferably, A/B 1.1. A readablereflectance difference is obtained when A/B 1.07. Herein, therefore, theetching satisfying A/B 1.07 is referred to as “selective etching”

The mean grain diameter of the particles constituting the sinteredsubstrate 50 is in the range between 0.3 μm and 0.5 μm, inclusive, whichis small compared with the width C of the concave portion formed bymarking (5 to 20 μm). Therefore, the bottom surface of the engravedconcave portion (etched surface 42) diffuses light emitted from anidentification information reader. This lowers the reflectance of thebottom surface compared with the reflectance of the flat portion(non-etched surface 41).

As described above, this embodiment of the present invention ischaracterized in that only a material as part of composite materialsconstituting the sintered substrate 50 is selectively etched, to lowerthe reflectance of the etched surface 42. By providing a reflectancedifference for the surface of the sintered substrate 50, the reflectedlight has a sufficiently high contrast even if the depth of the concaveportion (etching depth) is reduced to the range between 5 nm and 200 nm,inclusive. The reflectance of the surface of the sintered substrate alsodepends on the wavelength or spectrum of the irradiated light. It istherefore necessary to secure a sufficiently large reflectancedifference for the irradiated light actually used by the opticalidentification information reader. A recognizable contrast can begenerated if the reflectance difference is 15% or more.

Marking Method for Sintered Product

First, a sintered substrate produced by a conventional method isprepared. In this embodiment, as the sintered substrate to be marked,used is a sintered product produced using a powder material composed ofat least two kinds of particles having different etching characteristicsagainst a specific etchant. As the material for such a sinteredsubstrate, preferred is a ceramic material essentially composed of twoor more kinds of compounds selected from the group consisting ofaluminum oxide, aluminum nitride, silicon oxide, silicon nitride,zirconium oxide, zirconium nitride, silicon carbide, titanium carbide,titanium oxide, and iron oxide.

An important point in this embodiment is that powder particles used forthe production of the sintered substrate exhibit two or more differentetching characteristics. For example, compounds having the samecomposition commonly expressed as Al₂O₃ may have greatly differentetching characteristics depending on whether or not an impurity such asa rare-earth oxide and an alkaline-earth oxide has been added and, ifadded, on the concentration of the impurity. It is therefore possible toprovide different etching characteristics for first and second powdersof which basic compositions are commonly expressed as Al₂O₃. Therefore,the marking method of this embodiment is also applicable to a sinteredsubstrate produced from two or more kinds of Al₂O₃ powders havingdifferent etching characteristics.

A preferred embodiment of the present invention will be described withreference to FIGS. 6A through 6D, using the sintered substrate 50 madeof an Al₂O₃—TiC ceramic material essentially composed of two kinds ofcompounds, Al₂O₃ and TiC, as an example.

Referring to FIG. 6A, before the selective etching, the area of thesurface of the sintered substrate 50, other than the portion to beetched, is masked with a mask layer 51 made of an etching-resistantmaterial. For example, if a positive photoresist for G-line is used asthe mask layer 51, the mask layer 51 is formed by applying a positivephotoresist for G-line (film thickness: 1 to 2 μm) to the surface of thesintered substrate 50 with a spinner or the like and baking thephotoresist. As this type of photoresist material, OFPR-800 of TokyoOhka Kogyo Co., Ltd. may be used. After the baking, the photoresist isirradiated with G-line of about 200 mJ/cm² via a photomask or titlerthat defines a pattern of identification information (light exposurestep). After the light exposure step, the resultant substrate isdeveloped, to form the resist mask 51 having an opening (width C)defining a pattern corresponding to the pattern of identificationinformation on the sintered substrate 50.

The resultant sintered substrate 50 with the resist mask 51 formedthereon is put in a reaction chamber (not shown) of an etching apparatusfor the selective etching. This selective etching may be dry etching orwet etching. If dry etching is adopted, it must be reactive etching inorder to effect the selective etching. The kind of etching gas as asource of the etchant may be appropriately selected depending on thematerial to be etched. For the Al₂O₃—TiC ceramic substrate, CF₄ gas andSiH₄ gas can be suitably used.

Once the selective etching is finished, the portion of the surface ofthe sintered substrate 50 that is not covered with the resist mask 51(etched surface 42) has a fine concave/convex profile as shown in FIG.6B.

Referring to FIG. 6C, the resist mask 51 is removed from the sinteredsubstrate 50. The reflectance R1 of the etched surface 42 having a fineconcave/convex profile is lower than the reflectance R2 of therelatively smooth non-etched surface 41 (R1<R2). FIG. 6D is a plan viewschematically illustrating the concave/convex etched surface 42.

The layout of the portion to be etched is defined by the plane patternof the resist mask 51. The plane pattern of the resist mask 51 is givenan arbitrary shape by the photomask or titler used in the light exposurestep. In this way, arbitrary identification information in the form of acharacter, a code, a barcode, or the like can be written on the sinteredsubstrate 50.

In order to reduce the reflectance R1 of the etched surface 42, the linewidth of an engraved character, code, barcode, or the like is preferablymade larger than the mean grain diameter of the particles constitutingthe sintered substrate 50.

In order to increase the reflectance R2 of the non-etched surface 41,the plane roughness of the surface 41 is preferably lessened to smooththe surface 41. To realize this, it is effective to polish the surfaceof the sintered substrate 50 before the etching.

As the reflectance variation, that is, the reflectance differenceΔR(R2−R1) is larger, the contrast is higher in the reflected light fromthe surface of the sintered substrate 50 that has been irradiated withlight. In the example shown in FIG. 6D, the etched surface 42 isobserved darker than the other area, which makes it easier to read theidentification information such as an engraved character correctly.

As described above, in this embodiment, the contrast is improved byforming the fine concave/convex profile on the etched surface withoutincreasing the etching depth. This minimizes the possibility that theengraved portion may serve as a source of dust particles.

Method for Fabricating Magnetic Head Substrate

An embodiment of the method for fabricating a magnetic head substrate ofthe present invention will be described.

First, an Al₂O₃—TiC ceramic substrate, for example, is prepared.Identification information is inscribed on the surface of the substrateopposite to the surface on which magnetic thin films are to be formed,that is, the back surface of the substrate, by the marking methoddescribed above. The depth of the inscribed grooves is controlled in therange between 5 nm and 200 nm, inclusive.

An amorphous aluminum oxide film (thickness: 0.5 to 20 μm, for example)is deposited on the front surface of the substrate The surface of theamorphous aluminum oxide film is then smoothed. Thus, the basic stage ofthe process for fabricating a substrate for a thin film magnetic head iscompleted.

Thereafter, various magnetic thin films are formed on the substrate fora thin film magnetic head. The resultant substrate is divided into aplurality of bars 20 as shown in FIG. 1C. Each of the bars 20 obtainedfrom the substrate has a plurality of marks of identificationinformation 13 as shown in FIG. 1B. Each of the magnetic head sliderobtained from each bar 20 has the identification information 13 uniqueto the magnetic head slider as shown in FIG. 1A. In this way, the serialnumber of each magnetic head slider can be easily identified. This makesit possible to perform the process management during production ofmagnetic heads as is performed conventionally.

In this embodiment, a magnetic head having the following advantages canbe fabricated. That is, the problem of contamination generated duringthe marking step, is overcome. In addition, the magnetic head can befavorably used for a storage disc drive that should be protected fromdust contamination as strictly as possible. As a result, the productionyield of the magnetic heads improves, and the reliability of themagnetic recording/reproducing apparatus provided with the magnetic headimproves.

Which portion of the magnetic head should be subjected to the markingcan be determined arbitrarily. As for the portion to be marked, thepresent invention is not limited to the marked positions indicated inthis embodiment.

Data Storage Disc Drive

FIG. 7 illustrates a cross-sectional structure of a data storage discdevice (hard disk drive) 70 provided with magnetic heads withidentification information written by the above-described marking methodof the present invention.

The illustrated drive 70 includes three magnetic disks 72 having amagnetic recording layer (not shown), medium spacers 74 inserted betweenthe magnetic disks 72, an electric motor 76 for rotating the magneticdisks 72, magnetic heads 78 that come close to the magnetic disks 72 foreffecting recording/reproduction of data. The magnetic heads 78 arethose fabricated by the method described above, where identificationinformation is engraved on the surface thereof. Each of the magneticheads 78 is fixed to the top end of a support member 79 so as to beaccessible to an arbitrary track on the rotating magnetic disk 72. Anelectric/magnetic transducer element (not shown) provided on eachmagnetic head 78 executes read/write of data on/from the magneticrecording layer (not shown) of the magnetic disk 72. The electric motor76 is fixed to a chassis 80 of the hard disk drive 70. A rotation column84 is mounted on the electric motor 76, so as to rotate the magneticdisks 72 together with the rotation column 84.

EXAMPLES

In the examples of this embodiment, a composite sintered Al₂O₃—TiCceramic substrate made of 66 wt. % of Al₂O₃ and 34 wt. % of TiC wasused. This substrate is a machined rectangular thin plate (50 mm×50mm)having a thickness of 1.2 mm with a surface thereof beingmirror-finished to have a surface roughness Ra of 0.5 to 1.5 nm usingdiamond slurry (mean grain diameter: 1 μm).

In the examples, identification information was inscribed on the backsurface of the substrate by etching the back surface under variousconditions. In this inscription, a 7-digit code (composed of sevencharacters and/or numerals) was marked for each head portion of thesubstrate. The identification information is not limited to a code, buta two-dimensional barcode may be marked. The detailed conditions of theetching are shown in Table 2 below.

A total of 3,000 codes (21,000 characters and/or numerals) were markedon the surface of one substrate. The size of each character/numeralengraved was about 100×150 μm.

TABLE 2 LPC Evalu- Etching Reflectance ation Depth Variation (pcs./Readout Example Method (nm) (%) 100 cc) rate (%) 1 ICP-RIE(CF₄) 70.222.25 3,578 96.5 2 ICP-RIE(CF₄) 33.6 23.75 3,879 99.4 3 ICP-RIE(CF₄)180.5 34.67 3,634 99.5 4 Cnv.RIE(CF₄) 69.4 65.63 8,956 99.6 5Cnv.RIE(CF₄) 37.5 46.25 6,765 99.2 6 Cnv.RIE(CF₄) 8.5 15.46 4,897 97.4 7Wet etch(HNO₃) 10.6 25.84 4,289 99.6

In Table 2, “ICP-RIE” stands for “inductively coupled plasma—reactiveion etching”, “Cnv.RIE” stands for “conventional reactive ion etching”,and “LPC” stands for “liquid particle counter”. The “etching depth”represents the mean depth of the etched surface having a fineconcave/convex profile. The “reflectance variation” represents thereflectance difference ΔR=R2−R1 for the light having a wavelength of 564nm. The “LPC evaluation” represents the number of particles existing inwater in which the ceramic substrate has been immersed after theetching, as measured with a particle counter (the number of particlesper 100 cc of water). The “readout rate” represents the value measuredwith a reader using the light having a wavelength of 564 nm asirradiated light. Instead of using this light, white light or othervisible light can be used for readout. As such a reader, “acuReader” ofKomatsu Ltd., for example, may be used.

For selective etching, a parallel plate type plasma etching apparatusprovided with a magnetic field generation coil was used in Examples 1 to3, while a parallel plate type plasma etching apparatus involving nomagnetic field application was used in Examples 4 to 6. In these etchingapparatus, an electrode on the substrate holder side thereof isconnected to a radio-frequency (RF) source, so that etching gas plasmais generated between upper and lower electrodes. In Example 7, theceramic substrate was immersed in an HNO₃ solution for selectiveetching.

In general, the ICP-RIE method can provide a high etching rate comparedwith the Cnv.RIE method because the former can attain a higher densityof electrons in the plasma. While the etching rates in Examples 1 to 3were in the range of 15 to 25 nm/min, the etching rates in Examples 4 to6 were in the range of 1 to 5 nm/min.

The selective etching was possible by adjusting the RF power, the gaspressure, the gas flow, and the like. For example, the etchingconditions in Example 2 were as follows:

RF power: 300 W, ICP power (power supplied to the magnetic fieldgeneration coil): 550 W, Gas used: CF₄, Gas pressure: 6 m Torr, Gasflow: 40 sccm, Etching rate: 16.8 nm/min., Etching time: 2 minutes.

Under the above conditions, selective etching as shown in FIG. 5B wassuccessfully performed. The etching conditions in Example 4 were asfollows:

RF power: 300 W, Gas used: CF₄, Gas pressure: 4 m Torr, Gas flow: 40sccm, Etching rate: 1.7 nm/min, Etching time: 40 minutes.

Under the above conditions, also, selective etching as shown in FIG. 5Bwas performed.

FIGS. 8 and 9 are SEM photographs of marking patterns obtained inExamples 2 and 4, respectively. As is apparent from these photographs,the etched portions are darker than the other portions, exhibiting animage with clear contrast.

Reading of the identification information is possible if the reflectionvariation ΔR for selective etching is 15% or more. From the standpointof obtaining an improved readout rate, the reflection variation A R forselective etching is preferably 20% or more.

If the etching depth is in the range between 5 nm and 200 nm, inclusive,both the readout rate and the LPC evaluation are in the practicallyusable levels. If further improvement in these properties is desired,the etching depth is preferably set in the range between 10 nm and 100nm, inclusive.

Comparative Examples

As comparative examples, non-selective etching was performed, in placeof the selective etching in the above examples, employing variousmethods shown in Table 3 below for the same substrates as those used inthe above examples.

TABLE 3 Etching Reflectance LPC Depth Variation Evaluation Readout Comp.Method (nm) (%) (pcs./100 cc) rate (%) 1 IBE (Ar) 62.5 1.25 2,879 35.0 2IBE (Ar) 25.3 0.00 3,178 5.0 3 ICP-RIE(CF₄) 353.8 35.87 24,587  99.6 4ICP-RIE(CF₄) 1.3 1.25 2,892 2.0 5 ICP-RIE(CF₄) 7.5 13.78 2,753 75.0 InTable 3, “IBE” stands for “ion beam etching”.

It is found in comparison with the examples shown in Table 2 that thesample exhibiting a high readout rate has a large number of particles,and that the sample having a small number of particles exhibits a lowreadout rate. In the ion beam etching employed in the comparativeexamples, the etching rate A₁ of Al₂O₃ and the etching rate B₁ of TiChave the relationship of 1.0≦A₁/B₁<1.07. This etching is therefore notthe “selective etching”.

FIGS. 10 and 11 are SEM photographs of marking patterns obtained inComparative Examples 1 and 5, respectively. In these comparativeexamples, a sufficient reflectance difference fails to be establishedbetween the etched portion and the non-etched portion. The etchingconditions in Comparative Example 1 were as follows:

Accelerating voltage: 500 V, Gas used: Ar, Gas pressure: 0.1 m Torr,Beam incident angle: 45°, Etching rate: 10 nm/min, Etching time: 6.5minutes.

The etching conditions in Comparative Example 5 were as follows:

RF power: 300 W, ICP power: 550 W, Gas used: CF₄, Gas pressure: 6 mTorr, Gas flow: 20 sccm, Etching rate: 15 nm/min, Etching time: 0.5minute.

In Comparative Example 5, although the same etching apparatus and thesame gas were used as those used in Example 2, the readout ratesignificantly lowered. The reason is presumably that due to the reducedgas flow the selective etching was incomplete in Comparative Example 5,thereby reducing the difference in etching rate between Al₂O₃ and TiC.

Thus, according to the marking method of one embodiment of the presentinvention, the inscription depth in laser marking is adjusted in therange between 0.1 μm and 5 μm, inclusive. This markedly reduces thenumber of dust particles generated and also minimizes formation of edgeburrs. By reducing the number of dust particles generated due to thelaser marking, inclusion of dust particles in a thin film can beprevented when the thin film is formed on a surface of the sinteredproduct after the laser marking. As a result, a high-quality thin filmwith reduced contamination can be formed on the surface of the sinteredproduct with high yield. The present invention is therefore suitablyapplicable to a thin film magnetic head substrate. In addition, byminimizing formation of edge burrs, the step of removing edge burrsafter the laser marking can be simplified. It is essential to smooth theinscribed surface when a sintered product is used, as the magnetic headsubstrate. According to the present invention, however, the smoothingstep may even be eliminated, which will greatly contribute toimprovement in productivity.

According to the marking method of another embodiment of the presentinvention, a contrast in reflection light sufficient for recognizing theidentification information is secured even if the recessed portionformed by marking is shallow. This significantly reduces the number ofdust particles generated from the marked portion without lowering therecognition ratio of the identification information. By reducing thenumber of dust particles generated due to the marking, inclusion of dustparticles in a thin film can be prevented when the thin film is formedon the surface of the sintered product after the marking. As a result, ahigh-quality thin film with reduced contamination can be formed on thesurface of the sintered product with high yield. The present inventionis therefore suitably applicable to a magnetic head substrate. Inaddition, since this method is evaded from formation of edge burrs thatare likely to be formed by laser marking, the step of removing edgeburrs after the marking can be simplified. It is essential to smooth theinscribed surface when a sintered product is used as the magnetic headsubstrate. According to the present invention, however, the smoothingstep may even be eliminated, which will greatly contribute toimprovement in productivity.

According to the present invention, generation of dust from the magnetichead as a complete final component is also reduced. This improves thereliability of the storage disc drive provided with the magnetic head.

While the present invention has been described in a preferredembodiment, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A marking method for a sintered product,comprising the steps of: preparing a sintered product formed of a powdermixture including first powder particles of a first material and secondpowder particles of a second material having an etching characteristicdifferent from an etching characteristic of the first material; andperforming selective etching for a selected portion of a surface of thesintered product, the selective etching including etching the firstpowder particles with priority over the second powder particles, therebyto write identification information on the surface of the sinteredproduct.
 2. A marking method according to claim 1, wherein the meangrain diameters of the first powder particles and the second powderparticles are in a range between 0.3 μm and 5.0 μm, inclusive.
 3. Amarking method according to claim 1, wherein the difference inreflectance between the portion of the surface of the sintered productthat has been subjected to the selective etching and a portion that hasnot been subjected to the selective etching is 15% or more for lighthaving a certain wavelength.
 4. A marking method according to claim 3,wherein the wavelength is included in a wavelength range for light usedfor irradiating the sintered product for optically reading theidentification information.
 5. A marking method according to claim 1,wherein the plane roughness of the portion of the surface of thesintered product that has not been subjected to the selective etching is5 nm or less.
 6. A marking method according to claim 1, wherein the meanetching depth of the portion of the surface of the sintered product thathas been subjected to the selective etching is in a range between 5 nmand 200 nm, inclusive.
 7. A marking method according to claim 1, whereinthe first material and the second material are compounds selected fromthe group consisting of aluminum oxide, aluminum nitride, silicon oxide,silicon nitride, zirconium oxide, zirconium nitride, silicon carbide,titanium carbide, titanium oxide, and iron oxide.
 8. A method forfabricating a magnetic head substrate made of a sintered product,comprising the steps of: preparing a sintered product formed of a powdermixture including first powder particles of a first material and secondpowder particles of a second material having an etching characteristicdifferent from an etching characteristic of the first material; andperforming selective etching for a selected portion of a surface of thesintered product, the selective etching including etching the firstpowder particles with priority over the second power particles, therebyto write identification information on the surface of the sinteredproduct.
 9. A method for fabricating a magnetic head, comprising thesteps of: preparing a sintered product formed of a powder mixtureincluding first powder particles of a first material and second powderparticles of a second material having an etching characteristicdifferent from an etching characteristic of the first material;performing selective etching for a selected portion of a surface of thesintered product, the selective etching including etching the firstpowder particles with priority over the second power particles, therebyto write identification information on the surface of the sinteredproduct; and fabricating a magnetic head from the sintered product. 10.A sintered product formed of a powder mixture including first powderparticles of a first material and second powder particles of a secondmaterial having an etching characteristic different from an etchingcharacteristic of the first material, wherein the first powder particlesare selectively etched with priority over the second power particles ina selected portion of a surface of the sintered product, thereby towrite identification information on the surface of the sintered product.11. A sintered product according to claim 10, wherein the mean graindiameters of the first powder particles and the second powder particlesare in a range between 0.3 μm and 5.0 μm, inclusive.
 12. A sinteredproduct according to claim 10, wherein the difference in reflectancebetween the portion of the surface of the sintered product that has beenetched and a portion that has not been etched is 15% or more for lighthaving a certain wavelength.
 13. A sintered product according to claim12, wherein the wavelength is included in a wavelength range for lightused for irradiating the sintered product for optically reading theidentification information.
 14. A sintered product according to claim10, wherein the plane roughness of the portion of the surface of thesintered product that has not been etched is 5 nm or less.
 15. Asintered product according to claim 10, wherein the mean etching depthof the portion of the surface of the sintered product that has beenetched is in a range of 5 nm and 200 nm, inclusive.
 16. A sinteredproduct according to claim 10, wherein the first material and the secondmaterial are compounds selected from the group consisting of aluminumoxide, aluminum nitride, silicon oxide, silicon nitride, zirconiumoxide, zirconium nitride, silicon carbide, titanium carbide, titaniumoxide, and iron oxide.
 17. A magnetic head substrate formed of asintered product made of a powder mixture including first powderparticles of a first material and second powder particles of a secondmaterial having an etching characteristic different from an etchingcharacteristic of the first material, wherein the first powder particlesare selectively etched with priority over the second power particles ina selected portion of a surface of the sintered product, thereby towrite identification information on the surface of the sintered product.18. A magnetic head comprising: a magnetic head substrate formed of asintered product made of a powder mixture including first powderparticles of a first material and second powder particles of a secondmaterial having an etching characteristic different from an etching tocharacteristic of the first material, the first powder particles beingselectively etched with priority over the second power particles in aselected portion of a surface of the sintered product, thereby to writeidentification information on the surface of the sintered product; andan electric/magnetic transducer element formed on the magnetic headsubstrate.
 19. A storage medium drive comprising: a magnetic headsubstrate formed of a sintered product made of a powder mixtureincluding first powder particles of a first material and second powderparticles of a second material having an etching characteristicdifferent from an etching characteristic of the first material, thefirst powder particles being selectively etched with priority over thesecond power particles in a selected portion of a surface of thesintered product, thereby to write identification information on thesurface of the sintered product; an electric/magnetic transducer elementformed on the magnetic head substrate; a storage medium having amagnetic recording film on/from which information is recorded/reproducedvia the magnetic head; and a motor for driving the storage medium.